In a manufacturing process of semiconductor devices or flat panel displays (FPDs), a plasma is used to perform a processing, such as etching, deposition, oxidation, sputtering or the like, so as to obtain a good reaction of a processing gas at a relatively low temperature. Conventionally, a capacitively coupled type plasma apparatus has been widely employed as a single-wafer plasma processing apparatus, especially, as a single-wafer plasma etching apparatus.
Generally, in the capacitively coupled plasma processing apparatus, an upper electrode and a lower electrode are disposed to face each other in parallel in a vacuum processing chamber, a substrate to be processed (a semiconductor wafer, a glass substrate or the like) is mounted on the upper electrode, and a radio frequency voltage is applied to either one of the upper and the lower electrode. Electrons are accelerated by an electric field formed by the radio frequency voltage to collide with a processing gas.
As a result of ionization by the collision between the electrons and the processing gas, a plasma is generated, and a desired microprocessing (for example, etching) is performed on the surface of the substrate by radicals or ions in the plasma. At this time, the electrode to which the radio frequency voltage is applied is connected with a radio frequency power supply via a blocking capacitor in a matching unit and thus serves as a cathode. A cathode coupling method in which the radio frequency voltage is applied to the lower electrode, serving as the cathode, for supporting the substrate enables an anisotropic etching by substantially vertically attracting ions in the plasma to the substrate with a self-bias voltage generated in the lower electrode (see, for example, Patent Document 1).
(Patent Document 1) Japanese Patent Laid-open Application No. H6-283474 & U.S. Pat. No. 5,494,522
In the conventional capacitively coupled plasma processing apparatus, an anode electrode to which no radio frequency power is applied is grounded. Typically, since the processing vessel is made of a metal such as aluminum or stainless steel and is frame-grounded, the anode electrode can be set to be at a ground potential via the processing vessel. For this reason, in case of a cathode coupling arrangement, the upper electrode serving as the anode electrode is built in the ceiling of the processing vessel to form a single body therewith, or the ceiling of the processing vessel itself is used as the upper electrode.
With a recent trend of miniaturization of design rules for the manufacturing process, a high-density plasma is required to be available at a low pressure for a plasma processing. In the capacitively coupled plasma processing apparatus as described above, the frequency of the radio frequency power tends to be gradually increased and a frequency of 40 MHz or greater is standardly used in recent years. However, if the frequency of the radio frequency power becomes high, a radio frequency current is made to be concentrated on a central portion of the electrode, so that a density of a plasma generated in a processing space between two electrodes becomes higher at the central portion of the electrode than that at the edge portion thereof. As a result, there occurs a problem that an in-surface uniformity of the process is considerably deteriorated.